June 5, 2012

Physicists searching for a hypothetical and rare process involving radioactive decay of atomic nuclei have gotten their most sensitive results to date, suggesting that these mysterious particles behave like other elementary particles at the quantum level.

If such a discovery can be made, the researchers said the process could have profound implications for how scientists understand the fundamental laws of physics and could help solve some of the biggest mysteries of the universe.

The latest results have shown the effectiveness of a new instrument -- the Enriched Xenon Observatory 200 (EXO-200) -- that researchers say will yield even greater discoveries. EXO-200 is an international collaboration led by Stanford University and the US Department of Energy´s (DOE) SLAC National Accelerator Laboratory and includes more than 80 researchers in all.

EXO-200 has already begun one of the most sensitive searches ever for “neutrinoless double-beta decay,” a mysterious mechanism in which two neutrinos, acting as particle and antiparticle, do not emerge from the nucleus.

If this decay were observed, it would signal that neutrinos have a different quantum structure than other elementary particles. But EXO-200 did not observe this decay, which establishes the strongest evidence yet that neutrinos behave like other particles.

In a normal double beta decay, which was first observed in 1986, two neutrons in an unstable atomic nucleus turn into two protons, in which two electrons and two antineutrinos are emitted in the process.

Through their experiments with EXO-200, physicists have narrowed down the range of possible masses for the neutrino, a tiny uncharged particle that rarely interacts with anything, zipping through the universe at nearly the speed of light, passing right through whatever it meets.

“The result could only have been more exciting if we'd been hit by a stroke of luck and detected neutrinoless double-beta decay,” said Giorgio Gratta, a professor of physics at Stanford University and spokesperson for EXO-200. “In the region where double-beta decay was expected, the detector recorded only one event. That means the background activity is very low and the detector is very sensitive. It's great news to say that we see nothing!”

However, physicists have also suggested that two neutrons could decay into two protons by emitting two electrons without producing any antineutrinos.

“People have been looking for this process for a very long time,” said Peter Vogel, senior research associate in physics, emeritus, at Caltech and a member of the EXO-200 team. “It would be a very fundamental discovery if someone actually observes it.”

Because a neutrino would eventually be produced in a single beta decay, two neutrinos that are produced in a neutrinoless double beta decay must somehow cancel each other out. For that to occur, a neutrino must be its own antiparticle, allowing one of the two neutrinos to act as an antineutrino and annihilate the other neutrino.

That a neutrino can be its own antiparticle is not predicted by the Standard Model -- the theory that describes how all elementary particles behave and interact. If this neutrinoless process does indeed exist, physicists would be forced to revise the Standard Model.

The process also has implications for cosmology and the origin of matter, said Vogel. When the Big Bang occurred, there were equal amounts of matter and antimatter. But somehow, the balance was tipped and matter began, slowly at first, taking a foothold and eventually led to the existence of all the matter in the universe, which now far outweighs the existence of antimatter. The fact that the neutrino can be its own antiparticle might have played a key role in tipping that balance, said the researchers.

More than a decade ago, a collaborative effort of the Heidelberg-Moscow Double Beta Decay Experiment, made a controversial claim that it had discovered neutrinoless double beta decay using germanium-76 isotopes. But the new data from EXO-200 makes it highly unlikely that the earlier results were valid.

The EXO-200 experiment, which started taking data last year, will continue its quest for the next several years.

At the core of the instrument is a thin-walled cylinder made of extremely pure copper. It is filled with roughly 440 pounds of liquid xenon and buried 2,150 feet deep at the DOE´s Waste Isolation Pilot Plant at a New Mexico salt bed where radioactive waste is stored. The xenon-136 isotope, which makes up the bulk of the xenon in the holding tank, is only one of very few substances that can theoretically undergo the decay.

This research was supported by the Department of Energy and National Science Foundation in the United States, the National Sciences and Engineering Research Council in Canada, the Swiss National Science Foundation in Switzerland, and Russian Foundation for Basic Research in Russia. The research used resources of the National Energy Research Scientific Computing Center.